Standardizing monthly egg survey data as an abundance index for spawning stock biomass of chub mackerel in the Northwest Pacific

We estimated egg abundances from monthly egg density data obtained by research surveys, which can be used as an abundance index of spawning stock biomass (SSB) for the Pacific stock of chub mackerel. We applied the vector-autoregressive spatio-temporal (VAST) model to the monthly egg survey data from 2005 to 2023 off the Pacific coast of Japan to cover the spawning ground of chub mackerel. This document provides important references and diagnostics on this standardization according to the “CPUE Standardization Protocol for Chub Mackerel”. The standardized CPUE reached its peak in 2019, but has been on a downward trend since then, reaching its lowest level in 2023 since 2005. Since we found no serious problems in the diagnostics of the spatio-temporal model, we suggest the estimated index can be used as an SSB abundance index for the forthcoming stock assessment of chub mackerel in the Technical Working Group for the Chub Mackerel Stock Assessment.